Circuit breakers are commonly used to protect branch circuits in residential and commercial buildings against overload and fault conditions. Basically, a circuit breaker includes a separable pair of electrical contacts, a spring-operated mechanism for effecting separation of the contacts, and a tripping mechanism upon the occurrence of the overload or fault condition. A representative circuit breaker is fully set forth in U.S. Pat. No. 2,889,428, issued to Kingdon et al., commonly assigned to the assignee herein and incorporated herein by reference.
An electric arc is produced each time the circuit breaker contacts are opened or closed. The detrimental effects from the arc on other internal components is most severe during interruption of the electrical contacts. An arc extinguishing mechanism is used to control and extinguish the arc and protect the other components of the circuit breaker.
For example, a common type of arc shield, which is placed in a recess or arc chamber of a circuit breaker is a series of spaced magnetic plates as illustrated in U.S. Pat. No. 2,811,607 issued to Dorfman et al. Another type of arc extinguishing mechanism is set forth in U.S. Pat. No. 2,898,427 issued to Nadeau, which discloses a one-piece u-shaped magnetic metallic member having a plurality of parallel slots with an arc runner portion to lead the arc to a venting passage. U.S. Pat. No. 2,429,722 to Jennings discloses an arc extinguisher using insulating side members mounted between the legs of u-shaped magnetic members and the side walls of the breaker casing. Another example is U.S. Pat. No. 4,616,200 issued to Fixemer et al. which discloses a molded arc barrier projecting into the arc chamber to shield the operating mechanism of the circuit breaker.
Circuit breakers that successfully protect internal components with an arc extinguishing mechanism at high fault levels sometimes fail at intermediate fault levels. The interrupting mechanism which creates the arc is different at high fault compared to intermediate fault levels. At intermediate fault levels, the arc may become immobile causing damage to internal components of the circuit breaker.
The need also arises to distribute more power through enclosures which are the same size or smaller. This requires increasing the electrical rating of the circuit breaker to carry same voltage and current density while decreasing the size of the enclosure housing the components like the arc extinguishing means.
Among the problems caused by increasing the electrical rating of a circuit breaker is the heat emitted by the arc created by interrupting the electrical contacts. Without dissipation of the arc and the heat build-up the other components of the circuit breaker will be damaged.